However, it is likely the party will try to make further demands, as well as refusing to support proposed changes to constituency boundaries that would make it easier for the Tories to win a majority at the next election.

The 25 per cent collapse in the size of the Lib Dems’ ranks members Mr Clegg’s party with fewer members than the British Psychologists Society or the population of the Northamptonshire town of Corby.

It is the first time the party’s membership has dipped below 50,000 since it was founded nearly 25 years ago.

Senior Lib Dems blamed the fall on “contamination” by association with the party’s Conservative Coalition partners.

The Coalition’s decision to raise tuition fees, reform the NHS and the shake-up the welfare system has dismayed the Lib Dems’ grassroots.

One of the party’s most senior figures said: “In the past we have been in opposition and we have attracted members who oppose Government policy.

“Now we are in Government and are having to make difficult decisions, many which have not exactly thrilled our supporters or ourselves. It is very hard to pick up members in such circumstances.”

Mr Clegg has stressed that the Lib Dems have achieved a great deal in power, including raising the point at which people start to pay income tax to nearly £10,000, boosting the pupil premium and promoting gay marriage.

Speculation about Mr Clegg’s leadership has grown in recent weeks, after Vince Cable, the Lib Dem Business Secretary, suggested that he would consider becoming the leader.

Membership of all political parties has fallen in recent years. In 1951 the Conservatives had nearly 3 million members.

The party does not publish exact figures, but recent estimates have suggested that Tory membership may have fallen to as low as 130,000.

Labour’s membership has increased under Ed Miliband’s leadership, but in the last year has remained static at just over 193,000.

Bob Russell, the Lib Dem MP for Colchester, said: “Falling membership is a big issue for all parties. It’s no longer fashionable to become a member. Often you find that local caravan clubs have more members that constituency parties.

“However, while I have noticed a decline in members there has been no decline in the number of activists. Mr and Mrs Bloggs may no longer be willing to become a member, but they are still happy to canvass and post leaflets.”

Donations to parties were lower in 2011 after rising strongly in the run-up to the general election the year before. Lib Dem donations fell from £4.6 million to £1.7 million. Pledges to the Conservatives halved from £30.7 million to £14.8 million.